1. Field of the Invention
The present invention generally relates to fiber reinforced composites and methods for their manufacture and, in particular, to fiber reinforced composites which incorporate recycled thermoplastic.
2. Description of the Related Art
Thermoplastic reinforced with random glass mat (also referred to as glass mat thermoplastic or GMT) is sold commercially in significant quantities (more than 200 million pounds per year). GMT is usually supplied as semi-finished sheets which are heated and compression molded to desired conformations. GMT is a very tough product with moderate stiffness that is used extensively by the automotive and construction industries. Typical automotive applications include battery trays, seats, station wagon loading doors, bumper beams, and sound shields.
GMT sheets are often commercially manufactured by a melt impregnation process. The glass fiber is typically either a swirled mat of continuous fiber or chopped fibers held together in a mat by a binder. GMT typically contains 20-50% by weight glass, more commonly about 30-40%. The matrix thermoplastic is commonly polypropylene (PP) although other thermoplastics are also used, such as polyamide-6,6 (nylon 66) and polyamide-6 (nylon 6) and polyethylene terephthalate (PET). Products are formed from GMT sheets by compression molding, a process wherein sheets of GMT are layered to a desired thickness, heated and placed in or on a mold, and pressure is applied to shape the sheets into the shape of the mold.
Recycled thermoplastic can be derived from many sources. One of the more plentiful and less expensive sources is carpet, such as from manufacturing waste or pre or post consumer waste. Whole carpet waste is produced during manufacture from unsold merchandise, and from post consumer disposal. Typically, whole carpet comprises nylon, polypropylene or PET pile or tufts, at least one backing formed from one or more polyolefins, such as polypropylene, and an adhesive material of styrene-butadiene rubber (SBR) applied as a latex and filled with an inorganic filler such as calcium carbonate. If the carpet is obtained post use, it may also contain some amount of dirt. A typical carpet sample has a pile weight of about 40 oz. per square yard, a backing weight of about 8 oz. per square yard and an adhesive weight (SBR latex and filler) of 24 oz. per square yard. In other words, a nylon (or PET) carpet sample may comprise about 56% nylon (or PET), about 11% polypropylene, and about 33% SBR plus filler.
Single component carpet waste results during manufacture when the upper sections of the carpet tufts are trimmed off, producing a waste material referred to as carpet lint or shear lint. Nearly pure single component waste can also be produced from separation processes, such as a separation process employed to recapture nylon. In such a process large amounts of polypropylene or other backing material may be recovered and there exists a need to find a suitable use for this material. Also, fiber waste is generated from the residual material left on tufting machines, from fiber spinning and from yarn preparation. It has become apparent in recent years that there is a need for finding uses for this, otherwise, discarded material.
Carpet recycling to form a thermoplastic composition is taught by David et al. in U.S. Pat. Nos. 5,294,384 and 5,591,802. The composition taught by David et al. is prepared by melt blending the carpet sample in an extruder that intensively mixes the components through high shearing stresses. The melt blend can then be cut into pellets, for example, or another useful and commercially acceptable form. It is taught that the melt blend can be used in applications requiring virgin thermoplastic material.
An article by Hugh C. Gardner of Amoco Fabrics and Fibers Company published in IFJ (August 1995), page 36, entitled xe2x80x9cCarpet Recycling Technology,xe2x80x9d discusses compression molding two pieces of carpet with a glass mat to obtain a laminate structure. Gardner teaches the use of whole pieces of carpet which is impracticable because it requires discrete placement of the carpet and glass mat. Furthermore, Gardner""s laminated structure would not lend itself to the normal mode of processing GMT, which is flow molding under compression, because the resin in the core would squirt out to fill the mold and uniform spreading of the glass fibers would not be achieved.
GE Plastics in Pittsfield, Mass. sells a continuous strand glass fiber reinforced polypropylene sheet under the trademarks AZDEL(copyright) SM10400 and AZDEL(copyright) PM10400. The sheet has a glass fiber content of about 40% by weight with the remainder being polypropylene resin. Other grades of AZDEL(copyright) are sold with different glass contents. However, product literature on AZDEL(copyright) does not teach or suggest the use of recycled thermoplastic or the use of nylon as the thermoplastic.
Therefore, there is a need to address these and other shortcomings of the prior art.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention is generally directed to fiber reinforced thermoplastic composites that incorporate recycled thermoplastic, and methods for their manufacture. Without resorting to expensive separation and cleaning operations, recycled thermoplastics exhibit inferior properties compared to virgin thermoplastics. This degradation is more pronounced when the recycled thermoplastic contains incompatible polymers, which is usually the case. Specialty polymers, which can xe2x80x9ccompatibilizexe2x80x9d incompatible polymers, are sold to address this problem. In accordance with an aspect of this invention, however, reinforcing fibers are added to recycled thermoplastic to enhance certain properties (usually strength, stiffness and impact resistance) without the necessity of utilizing expensive separation and cleaning operations. Enhancement of the recycled thermoplastic composite is realized because the reinforcing fibers dominate the performance of the composite.
In a preferred embodiment, the thermoplastic composite incorporates a matrix of recycled thermoplastic and a plurality of high modulus fibers, with the recycled thermoplastics including polypropylene, nylon, PET and styrene-butadiene rubber, and the high modulus fibers including glass fibers, carbon fibers, natural fibers and aramid fibers. Such high modulus fibers have a modulus greater than one million psi, substantially greater than the unreinforced matrix. The recycled thermoplastic material and high modulus fibers are combined under low stress conditions. By avoiding typical high stress combination processes, such as encountered in extrusion compounding, substantial attrition of the length of the high modulus fibers is avoided, thereby preserving a long fiber length which imparts comparably high impact strengths to the composites. Preferably, each of the high modulus fibers have a minimum length of approximately xc2xd of an inch.
In accordance with another aspect of the present invention, thermoplastic composite is initially produced with at least a first component and a second component, with the first component incorporating recycled thermoplastic and the second component incorporating high modulus fibers. Preferably these components are immediately combined into a composite wherein the thermoplastic compound has merged with or intermixed with the reinforcing compound. In a preferred embodiment, the recycled thermoplastic is derived from carpet, with the high modulus fibers being present in the form of glass mat or continuous reinforcing fiber. It is also preferred that the composite formed as a continuous roll, strip or rod for collection and further configuration for injection or compression molding.
In accordance with another aspect of the present invention, an alternative method incorporates the steps of: (1) forming a carpet into particles or film of recycled thermoplastic material; (2) providing high modulus fibers; (3) applying the particles or film to the high modulus fibers; and (4) forming the particles or film and the high modulus fibers into a consolidated sheet. Preferably, the consolidated sheet is then compression molded to form shaped objects for use in various applications.
A method aspect of the present invention includes the steps of: (1) providing recycled thermoplastic; (2) providing high modulus fibers, with each of said high modulus fibers has a minimum length of approximately xc2xd of an inch; (3) combining the recycled thermoplastic with the high modulus fibers; and (4) forming the recycled thermoplastic and the high modulus fibers into a composite.
In a preferred embodiment, the step of providing recycled thermoplastic comprises the combination of the recycled thermoplastic in an extruder, whereby the recycled thermoplastic and the high modulus fibers are homogeneously mixed. Also, in this preferred embodiment, the high modulus fibers are provided in a continuous strand form, as opposed to chopped or pelleted form, for combination with the recycled thermoplastic in an extruder. As the combined thermoplastic and fibers are mixed before exiting an extrusion die, the fibers are substantially aligned in the process flow direction and are intermixed so as to form a homogenous composite upon exiting the extrusion die as continuous rods or strips. The continuous rods or strips are then chopped to at least a length of xc2xd inch pellets of composite to assure that the length of the reinforcing fibers within the composite pellets is of sufficient length to impart fiber reinforcement benefits.
In another preferred embodiment, the step of providing recycled thermoplastic comprises use of a plasticating extruder to melt the recycled thermoplastic followed by the addition of chopped high modulus fibers. The combined recycled thermoplastic and the high modulus fibers are passed through a low sheer extruder and extrusion die and formed into continuous logs or bars which can then either be rolled into continuous sheets or shaped forms or chopped into sections for compression molding applications. It is anticipated that the extrusion stage may be accomplished in one of several manners, including: use of a non-intermeshing, co-rotating twin screw extruder; a two stage screw extruder; or two screw extruders in series.
The numerous features and advantages of the present invention will be more readily apparent from the following detailed description read in conjunction with the accompanying drawings.